In a manner that is harmful to health or cleanliness; in ways that oppose good hygiene.
From antihygienic + -ally (adverbial suffix). The -ally suffix converts the adjective into an adverb describing how an action is performed, following standard English word-formation rules.
This word is rarely used because when something is antihygienic, we usually just call it 'unsanitary' or 'unclean'—language naturally prefers simpler words, so overly long technical terms often fade away.
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